Have to second this. Really enjoyed my visit to the Tesla Museum in Belgrade (and the rest of the former Yugoslavia in general-- jewel of Europe.)
If I remember correctly, those behind the museum had to work quite hard to get some of Tesla's paperwork back from the U.S. government after Tesla passed away. I have the feeling it was stuff that would have never seen the light of day again if it wasn't for their efforts.
Entrance is around 2.5EUR equivalent to $3.